1. Field of the Invention
The present invention relates to a sheet conveying apparatus, an image forming apparatus, a sheet conveying distance calculation apparatus and a sheet length calculation apparatus.
2. Description of the Related Art
In a commercial printing business, Print on Demand (POD) by an image forming apparatus using electrophotography instead of using an offset printing machine has been provided for printing small lots of data, various types of data or variable data has been increasing. In order to meet this kind of need, registration on both surfaces is required for the image forming apparatus using electrophotography comparable to that of the offset printing machine.
There are two main reasons for causing a registration error occurring in both-sides printing, including registration error in the lateral and the vertical directions, and a skew error between a sheet and an image. Further, for an image forming apparatus including a heat fixing device, an image size error caused by expansion and contraction of the sheet is also a reason for registration error occurring in both-sides printing.
In order to automatically correct the registration error in both-sides printing caused by the image size error, it is required to use a technique to automatically and accurately measure the size of a sheet, the conveying distance of the sheet or the like. Thus, a technique to measure the length of the sheet by detecting passing of a front end and a rear end of the sheet and calculating the length of the sheet based on the period between the passing of the front end and the rear end of the sheet, or the like is known.
In Patent Document 1, a length measuring means for measuring a length of an object to be transferred is disclosed. The length measuring means includes a rotating member that conveys the object to be transferred, a passing detection means that detects passing of the object to be transferred, a rotating amount measurement means that measures a rotating amount of the rotating member and a speed detection means that detects conveying speed of the object to be transferred. The length measuring means measures the length of an object to be transferred based on the rotating amount of the rotating member and the conveying speed of the object to be transferred.
According to Patent Document 1, it is described that the length of the object to be transferred can be measured by the length measuring means without being influenced by the decentering of a conveying roller or variance of diameter of the conveying roller.
In Patent Document 2, a sheet length measurement apparatus for measuring a length of a paper is disclosed. The sheet length measurement apparatus includes a length measuring roller, an upstream edge sensor and a downstream edge sensor respectively provided at upstream and downstream of the length measuring roller for detecting the position of the paper, and conveying rollers respectively provided between the length measuring roller and the upstream edge sensor and between the length measuring roller and the downstream edge sensor. The sheet length measurement apparatus measures the length of the paper based on the rotating amount of the length measuring roller.
According to Patent Document 2, it is described that looseness of the paper can be prevented from being generated by the conveying rollers so that the length of the paper can be measured based on the rotating amount of the length measuring roller which is being rotated while contacting the paper, by the sheet length measurement apparatus.
In Patent Document 3, a sheet length measurement apparatus that measures a length of a recording sheet is disclosed. The sheet length measurement apparatus includes a length measuring roller which is being rotated in accordance with the movement of a paper by contacting the paper which is being conveyed on a conveying path, an encoder device that detects a rotating amount of the length measuring roller, and an opposing roller which is positioned to face the length measuring roller such that the length measuring roller is rotated in accordance with the movement of the paper.
According to Patent Document 3, it is described that the length measuring roller is surely rotated in accordance with the conveying movement of the paper, and the sheet length can be measured by the sheet length measurement apparatus.
However, for the length measuring means disclosed in Patent Document 1, the speed detection means for detecting the conveying speed of the object to be transferred is necessary so that the structure of the apparatus becomes complicated.
For the sheet length measurement apparatus disclosed in Patent Document 2 or Patent Document 3, the conveying rollers are provided upstream and downstream of the length measuring roller on the conveying path of the recording sheet to cause the structure of the apparatus to be complicated. Further, as the length measuring roller does not have a driving force, there may be a case where slipping, looseness or the like is generated between the recording sheet and the length measuring roller so that it is not possible to accurately measure the sheet length.